In the field of deep foundations, and especially for repairing existing dams, there is the need of identifying a method and a device for forming impermeable diaphragm walls at great depths, in ground with high resistance, and that can ensure high accuracy and productivity.
In EP 0580.264 the guide pre-excavations are carried out with a pile machine. The miller is guided on two pre-excavations with tubular-shaped shields. The guide pre-excavations, however, must be very precise and in order to allow the milling body to have some clearance, one of the two guide shields is mounted on the articulated parallelogram with a spring system. In any case, if the shield moves far away a “dead area” is generated which is not excavated by the wheel.
Moreover, the hole is made by using a ballasted tool (35 tonnes plumb-lined with an upward pull of 10-15 tonnes), therefore if the pre-holes are not vertical, the tool guided by the holes would follow their profile and therefore it would not be possible to be certain that a perfect vertical hole would be made.
Moreover, since the guide shields are ballasted in their lower part and they extend beyond the excavation wheels, it is necessary to carry out the guide holes deeper with respect to the required depth.
Finally, the use of shape-tube during the casting of the panel so as to leave a guide hole free for the following panel poses practical construction problems which cannot always be solved.
This shape-tube has, on the side opposite the casting volume, an inflatable membrane so that the concrete hardens taking up the precise shape of the excavation guide. In the case in which there are very deep diaphragm walls, the use of a shape-tube can be impossible for practical reasons.
DE 1484545 describes a system for ensuring contiguous panels do intersecate. According to this patent, guide holes are formed at the joints between the panels through a pile machine. Subsequently, the bucket excavates between these, without necessarily having guide elements engaging in the holes, but simply exploiting the fact that the clam-shells remain in the area which has already been excavated since there is less resistance.
The aforementioned patent, however, does not describe a method or a device for solving the problem of the deviation of the guide piles.
In JP 59130920 two guide pre-holes are exploited at the joints of adjacent panels. The pre-holes are partially filled with low strength concrete; when casting these, tubes that remain hollow on the inside are introduced. The area which remains free constitutes the guides for two lateral shields mounted on the miller.
The tubes that are lowered inside the pre-holes, however, must be located in a very precise manner, and without external adjustment and fixing means (like mechanical or hydraulic tube centring means commonly called “plumb device”) it is very difficult for them to stay in position, especially during the casting. If the tubes are not kept perfectly vertical and parallel to one another, the following tool can encounter difficulties when proceeding inside the hole.
Moreover, the shields mounted on the miller body have jacks which are surely used to recover clearances in the guide and presumably to carry out small deviation corrections.
FR 19910004847 describes a method and equipment for guiding a tool for diaphragm walls for avoiding that two adjacent panels diverge.
The guide is a component that is inserted for the entire depth which is intended to be excavated. Inside this a filler is cast (low strength concrete or foams) which stabilizes the guide, whereas an area (in one case the central part, in another case two lateral areas) remains free so as to receive a guide element that is attached to the miller body. Through the control of the wheels a force is generated in the direction of the guide so as to ensure that the miller body remains adherent to the guide itself.
However, generating the force in the direction of the guide implies using the two wheels on the opposite side with higher revs than the others; this means that the entire productivity of the machine is not used.
Moreover, the use of a single guide on one side does not actually prevent the tool from rotating about the longitudinal excavation direction and does not therefore ensure the perfect alignment of contiguous panels. Moreover the guide has such a rectangular geometry that it is necessary to make the first panel (which is not a circular hole in this case) with an excavation technology that is intrinsically less precise (bucket, miller) thus presumably using the same tool, but not guided.
DE 1634323 describes a device (of the tube form type) to be inserted in the panel excavated at the guide pre-hole before casting the panel so that the pre-hole remains free for the following panel. This device has two mobile shells which can be adapted to the walls of the excavation.
The patent, however, does not describe a method or a device for minimising the problem of deviation of the guide piles.
DE 3823784 describes a method and a tool for obtaining narrow waterproofing panels, by using a trencher (chain cutting) guided by two uprights inserted in two holes previously obtained. The patent also describes the fact that the cutting edge of the chain is at an angle with respect to the horizontal.
As a matter of fact having a very wide angle on the cutting edge implies an increase in the surface to be excavated and thus a reduction of the productivity for the same installed power. Moreover, it makes it necessary to reach a greater excavation depth with respect to the height specified in the design.
In addition, having a single cutting chain means that the loads generated during the drilling are not balanced and tend to deviate the tool, thus increasing the risks of getting stuck due to the use of two guides simultaneously.
JP 58156630 describes a method and a device for making long diaphragm walls in a single step.
The system is of the horizontal trencher type. Two guide holes are made at the ends of the panel. The structures that contain a thrust system for the trencher, made with a rope actuated from outside and some diverter pulleys, are housed inside these holes.
The holes, however, will have a limited depth since it is necessary for the thrusted guiding and pulling structure to reach the depth required for the panel. The invention proposes to increase the field of application of the devices for making diaphragm walls in rock or high resistance ground, and at the same time ensure verticality of the panels within certain limits and their alignment.
In order to increase the excavation capability it is necessary to increase the weight of the excavating device, but as known this increases the deviations. Consequently it is necessary to use a guide system for minimising and/or correcting the deviations.
The prior art describes methods which are not optimal. Indeed, in some cases like those mentioned above EP 0580264A1, DE 1484545A1, JP 59130920A, DE 1634323B1, DE 3823784A, JP 58156630A methods are described that exploit two holes as guides at the ends of the panel, but these methods require the two holes to be extremely precise and parallel to one another. Indeed, considering depths in the order of 100 m, even when exploiting the most precise technologies, it is not possible to drop below deviations of 20 cm. Considering that the two guide holes can deviate in different directions, and thus diverge or converge, there is the risk of the tool getting stuck, of breaking or of a great drop in production.
The system proposed by the aforementioned patent EP 0580264 foresees using an articulated parallelogram system which does not solve the problem since, if the deviation of the guide holes is very strong, a “dead area” is generated in which the tool does not excavate, thus generating cusps which can make the tool become stuck.
Not even the aforementioned patent FR 19910004847 solves the problem. Indeed, by using a single guide on one side of the panel, the system does not in fact prevent the tool from rotating about the longitudinal excavation direction and therefore it does not ensure the perfect placing of contiguous panels over one another and their alignment along a direction specified in the design. Furthermore, the excavation that receives the guide system has a rectangular geometry such that it is necessary to make the first panel with an excavation technology which is not very precise, presumably with the same tool but not guided, thus obtaining a first panel which is not very precise, even though it is the most important one since it is that which will carry the guide. Such a system thus ensures the continuity of the diaphragm wall formed by the different elements, but not the accuracy in terms of verticality. Moreover, the device described in the aforementioned patent JP 58156630 is not suitable for reaching great depths since it requires the positioning of the structure in order to carry out the thrust to a maximum required depth. The latter moreover must be blocked from outside in order to exert its thrust.